A hydrometer measures the sugar in your wort or beer. Most pre-fermented wort will have a specific gravity ("SG") anywhere from 1.030 - 1.1, or more likely from 1.040 - 1.090. When it is fermented, that number goes down, depending on how much sugar was consumed. Different yeast strains and brewing processes and choices of fermentable ingredients will effect how much of the wort is fermented. Simple sugars (like dextrose/sucrose) ferment ~100% with any commercially-available brewer's yeast. Some ingredients, like crystal malt, will contribute a lot of sugars (like maltodextrin) that will not ferment out, making beer sweeter. Some common malt-based sugars, like maltotriose will be consumed by some yeasts and not others. Still, even those complex carbohydrates that can't be eaten by standard brewer's yeasts will be consumed by certain "wild" yeasts like brettanomyces, which are used in the production of a variety of specialty beers, such as the Belgian Lambics produced by Cantillon in Brussels.
In practice, if your beer is made with 4 kg dry malt extract and has a specific gravity of 1.062 and you ferment it with BRY-97 yeast, you will likely have a final gravity of 1.015. If you measure it and it is 1.016, then you wait two days and measure it again and it is still 1.016, it is done and you got 73% apparent attenuation (that's how much sugar it looks like your yeast ate). You can use those two measurements to determine the alcohol content, using a
calculator, such as this one. That beer that went from 1.062 to 1.016 has approximately 6.04% alcohol by volume ("abv").
Now, let's say instead that you used 3.6 kg of dry malt extract and 400 g of plain cane sugar (I think you call it castor sugar), that beer would have a SG of 1.062, the same as before, but the yeast would now eat all that cane sugar, in addition to 73% of the sugars in the dry malt extract. Your SG might be 1.062, but your final gravity will now be 1.015 and your abv will be 6.2%.
My advice to a newbie: measure your SG before you pitch your yeast and note it down somewhere. Then, when your beer appears to be finished fermenting (after a week or so), give it another week or two beyond that point. Then measure your gravity again when you are ready to bottle or keg.
